The usual microstructure of compacted graphite iron (CGI) consists of a matrix of ferrite and/or pearlite, which are formed from austenite, with a vermicular type of graphite dispersed throughout. In ductile iron (DI), the usual microstructure has a similar metal matrix with graphite spheroids or nodules dispersed throughout the structure. Both CGI and DI exhibit excellent castability, allowing the production of thin-wall castings from these iron types. However, the rapid cooling during solidification of these thin-wall castings makes it difficult to achieve the required microstructure in the as-cast condition, and hard, brittle carbides are nearly always present. As a result, the mechanical properties (e.g., hardness, ductility, toughness, etc.) are detrimentally affected as is machinability, limiting the successful production of thin-wall castings from CGI and DI.
The amount, size and distribution of vermicular and nodular graphite particles in the structure of the CGI and DI, respectively, are very important to the physical properties of the irons. The use of inoculants to control microstructure as well as to reduce the “chill tendency” or the formation of iron carbides (or cementite) is common practice in the ferrous foundry industry. The presence of iron carbides in the CGI or DI microstructure is undesirable because this constituent of the microstucture is hard and brittle and can result in poor mechanical properties (e.g., hardness, ductility, toughness, etc.) and machinability of the CGI and DI. Therefore, foundry metallurgical practices include the step of inoculating the metal so that nucleation and growth of the vermicular graphite or nodules occurs in a pattern that enhances the desired properties of the CGI or DI. The inoculating agent can be added to the pouring ladle, can be injected or sprayed in a finely divided or powdered form into the metal pouring stream as the molten metal enters the mold, or can be formed as an insert that is placed in the mold prior to pouring the molten metal into the mold.
The necessity of inoculation for different types of cast iron is determined by the thermodynamic nature of the molten iron as determined by the iron-carbon equilibrium diagram, which exhibits both stable (iron-graphite) and meta stable (iron-cementite) liquid-solid type transformations. Only a small temperature gap separates stable from meta stable iron solidification. Also, different kinetic effects and thermodynamic deceleration of the iron solidification can undercool a melt and promote cementite formation, i.e., the chill tendency. From a practical point of view, the main factors which increase chill tendency of a melt are the following:                a) increasing the rate of cooling during solidification in thin-wall castings;        b) the transformation of graphite to flake graphite and to vermicular and/or nodule shape (spheroidal) graphite; and        c) the presence of cementite-forming elements.All of these factors are present to some extent during thin-wall casting production using DI and CGI. The rapid cooling during solidification of thin-wall castings makes it very difficult to achieve the required structure in the as-cast condition and the mechanical properties and machinability of the casting are detrimentally affected, thereby limiting the successful production of thin-wall castings from DI and CGI.        
While nucleation or growth of eutectic cells in gray iron occurs when both the austenite and the flake graphite are in contact with the melt, this is not typical in DI where early in the solidification process the graphite spheroid is surrounded by an austenite shell. Vermicular graphite in CGI also has limited contact with melt. Therefore, direct contact with melt is lost and the spheroidal or vermicular graphite eutectic cell growth rate in DI and CGI is dependent on graphite diffusion through the austenite shell. The presence of the shell slows graphite diffusion and decreases the eutectic cell growth rate, consequently increasing undercooling and carbide formation in DI and CGI. Therefore, homogeneous nucleation will not occur unless an effective substrate, such as an inoculant, is present that will provide additional nucleation sites in the melt.
Inoculants can best be described as elements that can form stable compounds with sulfur and/or oxygen. These oxy-sulfide atomic clusters provide a substrate surface upon which dissolved carbon in the molten iron can “nucleate” or start to grow as graphite flakes or nodules to enhance desirable physical properties for the iron castings formed, before sufficient undercooling occurs that favors the formation of carbides which increase the hardness of the iron.
Numerous metal compositions and alloys have been proposed for use as inoculating agents in the production of both CGI and DI thin-wall castings. Standard inoculating agents are calcium silicon, calcium bearing ferrosilicon alloys or other ferrosilicon based alloys that contain small percentages of oxy-sulfide forming elements, and finely divided and powdered synthetic graphite.
As discussed previously, inoculants are commonly added to the molten metal in the pouring ladle prior to the actual solidification process. A major problem in using any of the above inoculants as a ladle addition is that the effectiveness of the inoculant diminishes rather rapidly after it is added to the metal. Thus, the first castings poured usually have improved microstructures and graphite structures versus those poured with metal from the same ladle only a few minutes later. This process of diminished effectiveness of inoculants with time at elevated metal pouring temperatures is known as inoculant fade. To circumvent or limit inoculant fade, some of the same inoculating alloys are used in a powdery or granular form and injected into the metal stream just prior to entering the mold. These methods are usually more effective and normally much smaller amounts of inoculant need to be added. However, mechanical problems associated with the actual injection process as well as the precise timing necessary for the injection of the inoculant powder into the metal stream may be the source of inconsistent results and contamination from un-dissolved inoculant particles.
Inoculating in the mold is a third alternative, although it is not widely used. In this inoculation method, either small lumps of calcium bearing ferrosilicon can be used or alternately, cast inserts made with ferrosilicon may be used. Further, since inoculation is performed essentially at the very last moment before solidification and virtually no time is available for fade in this method, even smaller amounts of inoculant may be used than are used when injecting the inoculant into the poured metal stream. However, efforts to make tablets with inoculant containing materials employing different binders have not met with commercial success. More recently, compacted and sintered fines of magnesium ferrosilicon and other silicon containing alloys have been also produced in the shape of a tablet for use as an in the mold inoculant, but still exhibit certain deficiencies.
In the manufacture of thin-wall CGI and DI castings, it is virtually essential to make an addition of an either a calcium bearing ferrosilicon or one of the more potent ferrosilicon inoculants containing relatively small percentages of oxy-sulfide forming elements prior to pouring the casting. In the case of the latter inoculant, these oxy-sulfide forming elements combine with dissolved oxygen and sulfur in the liquid iron. In almost all cases, the purpose of the ferrosilicon is to act only as a carrier for the oxy-sulfide forming elements and the ferrosilicon by itself provides little to no inoculating effect. Only certain amounts of these inoculating capable elements (or oxy-sulfide forming elements) can be technically and feasibly smelted and alloyed with the ferrosilicon to produce commercially and economically available alloy products. This is largely due to the limited solubilities of the oxy-sulfide forming elements in liquid ferrosilicon alloys. It should be mentioned that ferrosilicon is used as the carrier medium because ferrosilicon is relatively inexpensive and dissolves quite easily when added to cast irons, thereby liberating through dissolution in the molten iron the small amounts of elements that can react with dissolved oxygen and sulfur present in the melt.
One inoculant of this type is known by the tradename of Superseed or Stronsil. This inoculant is a strontium bearing ferrosilicon alloy containing small amounts of strontium (less than 1%) to promote the formation of graphite flakes and to minimize the formation of iron carbides. Other such ferrosilicon inoculants that contain strontium, calcium and either zirconium or titanium are disclosed in U.S. Pat. No. 4,666,516. Another titanium ferrosilicon alloy, this one containing magnesium is disclosed in U.S. Pat. No. 4,568,388. Finally, inoculating alloys for CGI are also known which include barium, e.g., U.S. Pat. Nos. 3,137,570 and 5,008,074.
The presence of alkali and/or rare earth metals in ferrosilicon inoculant compositions create extra nucleation sites for graphite by reacting with the soluble in-melt impurities of sulfur and oxygen. For example, inoculants combining ferrosilicon with barium, strontium, and/or calcium are effective for forming iron castings having flake graphite, as barium increases the time at which inoculant fade occurs, and strontium promotes graphite formation while minimizing iron carbide formation. However, for spherical and vermicular graphite formation in castings from DI and CGI, these cast irons are highly refined to remove practically all of the impurities present, i.e., sulfur and oxygen, by using a magnesium treatment. Thus, the alkali and rare earth metals in the conventional inoculants do not have the necessary oxygen and sulfur to react with and therefore cannot create additional nucleation sites. As a result, these types of inoculants are not very effective for thin-wall DI and CGI castings. Also, attempts to create extra nucleation sites in CGI and DI by putting into the melt preformed non-metallic substrates have not been effective and do not produce stable and uniform thin wall CGI or DI structures.
The reason for this is that most traditional inoculants do not contain intentional additions of sulfur or oxygen and must rely on the potential reaction of the oxy-sulfide forming elements which are added to traditional inoculants. Traditionally, all ferrosilicon based inoculants are smelted and refined in submerged arc furnaces and it is technically unfeasible to smelt sulfur and oxygen in combination with these alloys because of liquid solubility constraints. It is also difficult, if not impossible to incorporate significant amounts of these property enhancing elements, i.e., sulfur and oxygen, in traditional smelted ferroalloys.
The effectiveness of all inoculating agents is a direct function of the amount of sulfur dissolved in the molten irons and to a lesser extent, the amount of dissolved oxygen present. The ability of oxy-sulfide forming elements to form nuclei assisting substrates, i.e., oxy-sulfide atomic clusters, which in turn provide a similar crystalline surface onto which dissolved carbon atoms can precipitate from the liquid iron and grow is a necessary prerequisite for inoculation. Therefore, the ability to incorporate sulfur and oxygen containing elements in the inoculant used in the formation of thin-wall CGI and DI castings would insure that sufficient sulfur and oxygen are available for subsequent reaction with the oxy-sulfide elements added with or contained in inoculants. Addition of these sulfide and oxygen compounds would rejuvenate the molten iron and improve its responsiveness to inoculation.
To this end, inoculating additives containing oxygen and sulfur components that increase the effectiveness of inoculants used for thin wall castings have been developed, the most recent of which are disclosed in Skaland U.S. Pat. No. 6,102,983, Igarashi et al. U.S. Pat. No. 6,126,713 and Naro U.S. Pat. No. 6,293,988. Each of these patents discloses an inoculation additive for improving the effects of the inoculation of thin-wall cast iron that is formed of a powder including oxide and/or sulfides and other alkali metals for promoting the nucleation of graphite in the molten iron.
More specifically, Naro U.S. Pat. No. 6,293,988 discloses that to improve the effectiveness of inoculation, a ferrosilicon free inoculant is utilized. This inoculant is mechanically pressed into a tablet from a powdered mixture which is formed of 10 to 49% wt. % silicon, 7 to 20% wt. % calcium, 2.5 to 10% wt. % sulfur, 2 to 4% wt. % of oxygen, and 2.5 to 7.5% wt. % magnesium with the balance being iron, and is used as an in-mold inoculant.
Further, Igarashi et al. U.S. Pat. No. 6,123,713 discloses an additive for use in producing DI castings which contains: (a) fine particles of magnesium oxide, and (b) a graphite spheroidizing material or inoculant, with a weight ratio of component (a) to component (b) in the additive of between 0.0001:1 to 0.6:1.
Also, according to Skaland U.S. Pat. No. 6,102,983 an inoculant for the manufacture of iron with flake, compacted, or spherical graphite is disclosed that has a base formed of a ferrosilicon alloy and includes 0.5–10% wt. % calcium, and/or strontium, barium, cerium, or lanthanum, a first additive having 0.5–10% wt. % oxygen in the form of one or more metal oxides, and/or a second additive having 0.1–10% wt. % of metal sulfide, followed by agglomeration of the components with a binder to form the inoculant.
In each of the above-disclosed inoculant additives or modifiers, the included oxides and sulfides of alkali or rare earth metals can create extra substrates or sites for graphite nucleation during the iron solidification. However, the large amount of liquid-solid interface energy in DI or CGI that has been refined by the addition of magnesium makes it difficult to effectively distribute the oxides and sulfides of alkali and/or rare earth metals throughout the cast iron melt. More specifically, the oxides such as MgO, SiO2, CaO, TiO2, among others, which are disclosed in these patents, often do not melt or dissolve and have a tendency to agglomerate in the melt, even if they are used in the form of a fine powder. As a result, the effectiveness of inoculants formed with these types of oxide and/or sulfide components decreases, which makes it difficult to produce thin-wall castings of GCI or DI with the desired and substantially uniform structure. Further, in each of the above-disclosed inoculants, the inoculant does not contain a sulfur providing component necessary to promote nucleation, the inoculant contains significant amounts of calcium causing slag defect formation to readily take place in the castings or the inoculant includes an additional binder which also causes defects to form in the casting.
As a result, it would be desirable to develop an additive for an inoculant used in the production of thin-wall CGI and DI castings that has a simple and easy to formulate composition, and that also includes both oxygen and sulfur containing components to readily increase the available nucleation sites in the melt to form thin-wall CGI and DI castings having a substantially uniform structure with desirable mechanical properties. The additive should also be formed to have a composition that virtually eliminates the presence of any alkali or rare earth metal oxides or sulfides, or binders to avoid the defect formation problems associated with prior art inoculants and additives.